It is now recognized that an epigenetic mechanism, gene transcriptional silencing in association with promoter DNA hypermethylation, is a major mode of functional inactivation of tumor suppressor genes in cancer. This alternative to gene mutations has now been associated with a rapidly expanding growing list of genes important to virtually every cellular pathway central to normal cell renewal and differentiation, and to cancer development when pathway disruption is produced. There is building data, some from our laboratory, to suggest that this type of gene silencing can drive some of the very earliest steps in tumor development by fostering an abnormal expansion of cells (stem cells and/or committed progenitors) which are then at risk for subsequent tumor progression. There is, then, a great need from a biological perspective, and for translational implications in tumor diagnosis and therapy, to understand the molecular steps underlying the evolution of promoter hypermethylation during tumor progression and the cell populations involved. The purpose of this grant is to establish whether altered expression of genes which drive normal stern cell and/or committed progenitor cell phenotypes, and normal cell lineage commitment steps, may induce the evolution of abnormal epigenetic gene silencing in tumor progression. Examples of such events, some of which involve genes which function as long term transcriptional repressers with known downstream targets which are hypomethylated in tumors, include overexpression of the polycomb protein, Bmi-1, and sonic hedgehog and Wnt pathway activation proteins. We will develop model systems, utilizing tumor evolution from both embryonic and mature stem cell/committed progenitors, and within the framework of adult lineage commitment steps, to study this question. The upstream gene expression events and epigenetic gene silencing profiles will be compared for embryonic stem cells, committed progenitor cells, and normal cell differentiation compartments of selected cell renewal areas such as lung, mammary,and hematopoietic cell systems and tumors arising from these cell populations. Then, selected normal cell types will be engineered for over-expression of selected upstream candidate genes, such as Bmi-1 and sonic hedgehog, and the solution of gene silencing, including temporal events for the formation of transcriptional silencing chromatin, aberrant promoter DNA methylation, and evolution of the proteins complexes mediating these events, will be studied. Animal models will then be utilized to extrapolate our results to in vivo tumor progression settings.
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